Amine solution treatment of cathodically chromated metal surfaces



United States Patent 3,484,343 AMINE SOLUTION TREATMENT OF CATHOD- ICALLY CHROMATED METAL SURFACES Yoichi Kitamura, Yokohama, and Tsuneo Inui, Kudamatsu-shi, Japan, assignors to Toyo Kohan Co., Ltd., Tokyo, Japan, a corporation of Japan No Drawing. Filed July 13, 1965, Ser. No. 471,717 Claims priority, application Japan, July 13, 1964, 39/39,192 Int. Cl. C23b 5/06; C23f 17/00 US. Cl. 204-35 8 Claims ABSTRACT OF THE DISCLOSURE The properties of hydrated chromium oxide coating on metal surfaces are improved by rinsing the coating with water, immersing it in an aqueous solution containing at least one water soluble amino compound and then again rinsing it with water. The examples of said water soluble amino compound are alkanol amines, alkyl amines, aromatic amines, polyalkylene polyamines, hydrazine and its alkyl derivatives and alkali metal salts of amino carboxylic acids and immersion into solutions of these compounds is conducted at a temperature of 30-70" C. for 1 to 60 seconds.

The present invention relates to a process for improving the properties of the coating formed in the cathodic chromating of metal surfaces.

Cathodic chromating is a process in which hexavalent chromium ion derived from chromic acid, chromates or dichromates in a treating solution is electrolytically reduced, mainly to trivalent state, to form a protective coating of a hydrated chromium oxide complex on a cathode metal. In the present specification the expression hydrated chromium oxide coating is employed for the film formed by chromating.

An analysis of the formed film shows that it contains no other metals than chromium and does not consist of simple metallic chromium. The analysis also shows the film contains a very little amount of an anion which originally is present in the chromating solution as an addition agent.

When the wetted hydrated chromium oxide coating formed immediately after the chromating is dried at an appropriate temperature, the anion in the coating is fixed as a member of a co-ordinated complex structure of the coating. In a wetted state, however, the anion is not firmly fixed in the structure and behaves as if it adsorbs physically.

As shown in the specific examples below the conditions for a cathodic chromating process are quite different from those for chromium plating, dip chromating and anodic chromating. A coating formed in chromium plating is a simple metallic chromium and the one formed in dip chromating is mainly a chromium chromate and in some cases is a mixture of a chromate and an oxide of metal to be coated, while the one formed in anodic chromating is a mixture of a chromate and an oxide of metal to be coated.

In the cathodic chromating process various addition agents such as boric acid, phosphoric acid, sulfuric acid, disulfonie acid, nitric acid, acetic acid, or their respective salts and halogen compounds are added to the electrolyte which mainly contains hexavalent chromium ion derived from chromic acid, chromates or dichromates as described in US. Patents Nos. 2,733,199, 2,780,592, 2,769,774, 2,998,361, 3,032,487, 3,257,295, 3,288,691 and 3,296,100.

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Of these addition agents, such a strongly acidic and corrosive anion as sulfate ion, nitrate ion or chlorate ion is involved in the electrolytic reduction of hexavalent chromium ion, and is physically adsorbed in the hydrated chromium oxide film on metal surfaces. The adsorbed anion in the film is water soluble and is dissolved out from the film under highly humid conditions to accelerate the corrosion of metal surfaces.

As described above, the addition agent which gives a strongly acidic anion is necessary to form an excellent film in cathodic chromating but gives detrimental effects on the corrosion resistance of the formed film.

Therefore the adsorbed acidic anion should be removed from the film after the film is formed in cathodic chromating.

In order to remove the strongly acidic anion adsorbed in the hydrated chromium oxide film and to improve the corrosion resistance and lacquer adhesion property of the film, an immediate immersion of the cathodically chromated metal, immediately after chromating, in an aqueous solution containing at least one water soluble amino compound selected from the group consisting of alkanol amines, alkyl amines, polyalkylenepolyamines, hydrazine and its alkyl derivatives, alkali metal salts of ethylenediamine-tetraacetic acid and the similar chelating agents and aromatic amines, is most effective.

The following water soluble amino compounds are used in the present invention.

(1) Water soluble alkanol amines:

Monoethanol amine, diethanol amine and triethanol amine.

(2) Water soluble alkyl amines:

Methyl amine, ethyl amine, n-propyl amine, isopropyl amine, n-butyl amine, sec-butyl amine, n-amyl amine, dimethyl amine, diethyl amine, di-n-propyl amine, di-isopropyl amine, trimethyl amine and triethyl amine.

(3) Water soluble polyalkylene polyamines:

Ethylene diamine, diethylene diamine, diethylene triamine and propylene diamine.

(4) Hydrazine and its water soluble alkyl derivatives:

Hydrazine, sym-dimethyl hydrazine and unsym-dimethyl hydrazine.

(5) Alkali metal salts of ethylenediaminetetraacetic acid and of the similar chelating agents:

Lithium, sodium, potassium, rubidium and cesium salts of chelating agents selected from the following aminocarboxylic acids.

Ethylenediaminetetraacetic acid, nitrilotriacetic acid, cyclohexanediaminetetraacetic acid, hydroxyethyliminodiacetic acid, hydroxyethylethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, di (hydroxyethyl)- glycine, ethyletherdiaminetetraacetic acid, glycoletherdiaminetetraacetic acid and ethylenediaminedi-o-hydroxyphenylacetic acid.

(6) Water soluble aromatic amines:

Aniline, benzyl amine, pyridine, a-picoline, ,B-picoline, w'y-lutidine, u-a-lutidine and p-phenylene diamine.

These water soluble amino compounds can be used alone or in mixture.

The amine treatment is further effective for improving the lacquer adhesion property of an cathodically chromated metal, especially in the case of lacquer coating on the mechanically formed metal after degreasing the lubricating oil. The oil adsorbed on an cathodically chromated metal is difficult to degrease because of the atfinity between oil and hydrated chromium oxide. Imperfect degreasing, therefore, results in poor lacquer adhesion. This defect, however, can be remedied by the amine treatment according to the present invention, in which amino compounds adsorbed act as a suitable barrier for oil adsorption.

The conditions required for performing the process described in the present invention will be summarized as follows:

(1) Concentration of water soluble amino compounds g./liter 1-100 (2) Temperature C 30-70 (3) Dipping time seconds 1-60 The optimum concentration of water soluble amino compounds covers the range from 1 g./liter to 100 g./liter.

Generally, an increase in concentration results in a higher removal efficiency of the adsorbed anion from the formed film. However, when the concentration of water soluble amino compounds exceeds 100 g./liter, the hydrated chromium oxide film becomes yellow and the corrosion resistance and lacquer adhesion property are not improved. When the concentration is less than 1 g./liter, the process needs a longer period which is not practical in high speed commercial operation.

Of the conditions, an increase in temperature results in a higher removal efficiency of the adsorbed anion from the formed film, but in the actual operation, higher temperatures increase the evaporation loss of water and the decomposition of water soluble amino compounds. The operation preferably should be controlled at a temperature below 70 C. On the other hand, a decrease in temperature results in a lower removal efficiency of the adsorbed anion from the formed film, and at temperatures below 30 C., the process requires an impractically long period.

Therefore it is preferable to conduct the operation within the temperature range of between 30 C. and 70 C.

An increase in dipping time results in a higher removal efiiciency of the adsorbed anion from the formed film, but in concentrations of treating solution of l-lOO g./liter and at treating temperatures of 3070 C., the dipping time of 1-60 seconds may be sufiiciently effective for the improvement of the corrosion resistance and lacquer adhesion property.

When dipping time exceeds 60 seconds, the dissolution of the formed film is initiated and when the dipping treatment is of a very short time of less than 1 second the process under the conditions mentioned above cannot be practically performed.

An immersion treatment in hot water of 80-90 C. is also effective to remove the adsorbed anion, but the process requires an impractically long period.

When the film formed in cathodic chromating is dry, an immersion treatment in an amine solution is less effective because the dried film behaves as a hard barrier against the action of amines.

Therefore, the amin treatment should be immediately performed before drying the film formed in cathodic chromating.

The iron and steel sheets treated according to the present invention can withstand outdoor exposure tests for two months in winter and for days in summer and also resist salt spray tests for 48 hours without any sign of rust. They further resist humidity chamber tests in the relative humidity of 90% at 40 C. for one week without any sign of discoloration of the film. Of conventional chemical surface treatments, such a treatment as that of the present invention has not been presented heretofore in respect of this high level of corrosion resistance.

After applying a film of about 10 microns of modified alkyd or modified epoxy lacquer on steel sheet treated according to the present invention, the lacquered sheet was deeply drawn to the cup with a drawing ratio of 2.2 and stretch-formed in the cup wall to make a screw cup; when tested by adhesive tape, the lacquered side of the cup showed no adhesion loss. Furthermore, no adhesion loss of the lacquer was found on the cup wall, even when tested by adhesive tape after the cup was immersed in a boiling 3% sodium chloride solution for one hour.

In the case of lacquer coating on the mechanically formed metal after degreasing lubricating oil, superior lacquer adhesion was found in the cathodically chromated metal treated according to the present invention as described in the following examples.

Metal surfaces treated according to the present invention showed good heat resistance up to 300 C., and weldability as good as base metals.

As metals to be treated according to the present invention, the conventional metals for cathodic chromating under ordinary conditions, such as iron, steel, aluminum, chromium, nickel, zinc, tin, their respective alloys and metals having their surfaces are suitable.

The examples of the present invention are as follows:

EXAMPLE 1 A 0.25 mm. cold rolled low carbon steel sheet, the so called black plate is cathodically cleaned for 20 seconds at a current density of 4 a./sq. dm. at 70 C. in a 7% sodium hydroxide solution, then rinsed with water, pickled for 10 seconds at room temperature in a 7% sulfuric acid, again rinsed with water and immediately treated cathodically using a lead antimony alloy anode under the following condition in the chromating electrolyte of the following composition, immediately rinsed with water, and immediately dipped in the treating solution of the present invention under the conditions given below, and again immediately rinsed with water and dried.

(1) The electrolyte and the conditions of electrolytic chromating.

Chromic acid g./liter 50 Ethyl alcohol do 0.2 Sulfuric acid do 0.2 Hydrofluosilicic acid do 0.3 Temperature C 50 Current density a./ sq. dm.-- 20 Time seconds 10 (2) The treating condition of the present invention.

Monoethyl amine g./liter 18 Temperature C 70 Dipping time seconds 30 A transparent, bluish purple film was formed and the steel sheet thus treated showed very little rusty spots after being subjected to a salt spray test with a 5% sodium chloride solution for 48 hours at 35 C. and it showed no rusty spot and no discoloration when subjected to a humidity chamber test with a relative humidity for one Week at 40 C.

EXAMPLE 2 A 0.5 mm. cold rolled low carbon steel sheet was subjected to the same pretreatment and the same electrolytic chromating, as described in Example 1, and then immediately rinsed with water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with Water and dried.

Disodium salt of ethylenediaminetetraacetic acid g./liter 1 Temperature C 60 Dipping time seconds 30 The film formed was bluish purple and transparent. When the steel sheet thus treated was applied with about 30 rug/sq. dm. of machine oil No. 120, degreased after 24 hours with trichloroethylene vapor for 2 minutes, and then coated with about 20 microns of melamine modified alkyd enamel and subjected to a cross cut test, no adhesion loss of the enamel was found. Furthermore when this enamel coated sheet was immersed in a boiling 3% sodium chloride solution for one hour, the enamel film did not show adhesion loss.

EXAMPLE 3 Chromic acid g./liter 60 Phenol-2,4-disulfonic acid g./liter 0.6 Temperature C 50 Current density a./sq. dm 20 Time seconds 20 (2) The treating condition of the present invention Triethanol amine -g./liter 100 Temperature C 50 Dipping time seconds 2 The film formed was bluish yellow and transparent and the steel sheet thus treated was found to have almost no rusty spots when subjected to the same salt spray test'as in Example 1 for 48 hours and also when the steel sheet treated was coated with about 20 microns of melamine modified alkyd enamel and subjected to the same salt spray test as in Example 1 for 150 hours no blisters of the enamel were found.

EXAMPLE 4 The same kind of cold rolled low carbon steel sheet was subjected to the same pretreatment as described in Example 1 and then immediately subjected to the same electrolytic chromating as described in Example 3, then immediately rinsed with water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water and dried.

Ethylene diamine g./liter.. 45 Temperature C 50 Dipping time seconds 60 The film formed was bluish yellow and transparent and the steel sheet treated had the similar corrosion resistance to those in Example 1. Furthermore when the steel sheet treated was coated with about 10 microns of modified alkyd white enamel and deeply drawn to the cup with a drawing ratio of 2.2 no adhesion loss of the enamel was found on the cup well.

EXAMPLE 5 The same kind of cold rolled low carbon steel sheet was subjected to the'same pretreatment as described in Example 1 and then immediately treated cathodically using a lead antimony alloy anode under the following condition in the chromating electrolyte of the following composition, again immediately rinsed with water and immediately dipped in the treating solution of the present invention under the following condition and again immediately rinsed with water and dried.

(1) The electrolyte and the condition of electrolytic chromating.

Sodium dichromate g./liter 500 Chromium sulfate g./liter Ammonium sulfate g./liter Temperature C-.. 50 Current density a./sq. dm Time seconds l0 6 (2) The treating condition of the present invention Hydrazine g /liter 50 Diethylene triamine g./liter 40 Temperature C 60 Dipping time .seconds 10 The film formed had the similar corrosion resistance to those in Example 1 and had the similar enamel adhesion property to those in Example 2.

EXAMPLE 6 Pyridine g./liter 20 Temperature C 30 Dipping time --seconds 40 The film formed was colorless and transparent, and

when the aluminum sheet thus treated was subjected to a porosity test by cupric sulfate solution for two hours, no porosity was found.

What we claim is:

1. A process for treating a cathodic chromated metal in which the chromated metal, before being allowed to dry, is rinsed with water and to remove from the chromated layer entrapped ions selected from the group consisting of sulfates, nitrates and chlorates is immersed in an aqueous solution containing a total of from 1 to grams per liter of at least one water soluble amino compound selected from the group consisting of alkanol amines, alkyl amines, aromatic amines, polyalkylene polyamines, hydrazine, alkyl derivatives of hydrazine and alkali metal salts of amino canboxylic acids at a temperature of from 30 to 70 C. for a period of from 1 to 60 seconds, and is thereafter rinsed again with water, the chromated metal being thereafter dried.

2. A process according to claim 1 wherein the water soluble alkanol amines are selected from the group consisting of monoethanolarnine, diethanolamine and triethanolamine.

3. A process according to claim 1, wherein the water soluble alkyl amines are selected from the group consisting of methyl amine, ethyl amine, n-propyl amine, isopropyl amine, n-butyl amine, sec-butyl amine, n-amyl amine, dimethyl amine, diethyl amine, di-n-propyl amine, di-isopropyl amine, trimethyl amine and triethyl amine.

4. A process according to claim 1, wherein the water soluble polyalkylene polyamines are selected from the group consisting of ethylene diamine, diethylene diamine, diethylene triamine and propylene diamine.

5. A process according to claim 1, wherein hydrazine and its Water soluble alkyl derivatives are selected from the groop consisting of hydrazine, symdimethylhydrazine and unsymdimethylhydrazine.

6. A process according to claim 1, wherein the aminocarboxylic acids are selected from the group consisting of ethylenediaminetetraacetic acid, nitrilotriacetic acid, cyclohexanediaminetetraacetic acid, hydroxyethyliminodiacetic acid, hydroxyethylethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, di-(hydroxy-ethyD- glycine, ethyletherdiaminetetraacetic acid, glycoletherdiaminetetraacetic acid, and ethylenediaminedi 0 hydroxyphenylacetic acid.

7. A process according to claim 1, wherein the water soluble aromatic amines are selected from the group consisting of aniline, benzyl amine, pyridine, otpiC0line, B- picoline, u,'y-lutidine, a,ot-lutidine and p-phenylene diamine.

References Cited UNITED STATES PATENTS Riou et a1. 1486.l4 Keller et al. l48-6.15

Uhlig 96-1 Nishigski et a1 20456 JOHN H. MACK, Primary Examiner W. B. VANSISE, Assistant Examiner 2,333,206 11/1943 Sloan 117 49 3,296,106 1/1967 Smith et a1. 204-56 3,337,431 8/1967 Kitamura et a1 204-56 10 US. Cl. X.R. 

